1. Field of the Invention
This invention relates generally to a charging system for automotive vehicles and, more particularly, to an apparatus and method for controlling low engine idle RPM without discharging a vehicle battery by continuously monitoring the vehicle alternator field modulation.
2. Discussion of the Related Art
In conventional vehicle charging systems, an to alternator, driven by an internal combustion engine, converts mechanical power into electrical power. This power is then used to charge the vehicle battery and drive various electrical systems or loads in the vehicle. As the speed or revolutions per minute (RPM) of the engine varies, the rotation speed of the armature in the alternator also varies, thereby changing the amount of current supplied by the alternator. Thus, with a high rotation speed, higher current can be supplied by the alternator and conversely with a lower rotation speed, lower current is generally supplied by the alternator. The modulation of the field coil in the alternator also varies the amount of current supplied by the alternator. In this way, with a higher modulation or duty cycle, the field coil is energized for a longer period of time, thereby providing a greater current supply compared to when the field coil is only energized for shorter periods of time.
To aid in improving vehicle fuel economy and reduce vehicle emissions, it is generally desirable to maintain the engine idle RPM as low as possible. Unfortunately, low idle speeds may not turn the alternator at a sufficient speed to provide an adequate current supply when the vehicle is heavily loaded electrically. To ensure that the vehicle battery does not become discharged during such low engine idle speeds, the voltage at the battery is generally monitored to ensure that the voltage does not drop below a predetermined value. If the voltage does drop below the predetermined value, the modulation to the field coil of the alternator is increased or the engine idle RPM is increased so that the alternator can supply more current to the battery to maintain the predetermined voltage. By monitoring the voltage at the battery and controlling either the engine RPM and/or the field coil modulation, discharge of the vehicle battery can generally be prevented.
However, such voltage monitoring systems have several disadvantages. For example, it is generally difficult to accurately monitor the battery voltage since the battery can generally be considered and modeled as a capacitor. Because of this, there is a delay in the voltage drop during actual discharge of the battery and, therefore, battery discharge may occur before it is accurately detected. In other words, an accurate corresponding voltage change does not occur because of the capacitance effect in the battery. Moreover, the battery voltage will generally vary significantly based on temperature and other conditions which makes monitoring such a reference even more difficult. This may result in the engine RPM being cycled high and low (in its attempt to maintain voltage) intermittently, which is an undesirable condition. Such a system also exhibits the inability to vary the engine RPM at the absolute minimum necessary to maintain the battery in a charge condition, thereby further increasing fuel consumption and vehicle emissions.
Several systems have tried to improve upon the above-identified deficiencies such as U.S. Pat. No. 4,794,898 to Kato. However, this system also exhibits several disadvantages. Specifically, Kato indicates that it detects both the conduction rate of a switching circuit which controls field current, as well as engine speed to determine the optimal engine idle speed. However, Kato actually averages a specifically monitored voltage output utilizing a separate smoothing circuit and utilizes this average voltage as a representation of the conduction rate. This significantly reduces the response time in such a system. In addition, Kato must utilize a look-up table generated from empirical data to compare the average voltage with a predetermined desired voltage at the detected RPM. If the average voltage does not match the desired voltage, the rotation speed of the engine is adjusted accordingly. This provides for a very labor-intensive and complex device which requires the gathering of empirical data for each engine to generate the look-up table, thereby making such a system extremely complex to implement and not very practical.
What is needed then is an apparatus and method for controlling low engine idle RPM without discharging a vehicle battery by monitoring the vehicle alternator field modulation. This, in turn, will improve vehicle fuel economy; reduce vehicle emissions; accurately and responsively determine battery discharge conditions; provide real time control and response based on internally or automatically knowing the modulation of the alternator field coil; reduce high/low cycling of engine RPM; and enable the engine to operate at the absolute minimum RPM necessary without battery discharge occurring. It is, therefore, the object of the present invention to provide such an apparatus and method for controlling low engine idle RPM without discharging a vehicle battery by monitoring the vehicle alternator field modulation.